Emission control with respect to PM (particulate matter), NOx, CO, HC and the like, discharged from the diesel engine has been increasing year by year. A mere improvement of the engine is no longer sufficient to achieve the regulation values which have been getting more severe accompanied with the enhanced control. The technique for reducing the aforementioned substances discharged from the engine has been introduced by mounting an exhaust gas after-treatment device in the exhaust passage of the engine.
In consideration of the above-described circumstances, various studies and proposals have been made with respect to the NOx catalyst for reducing and removing the NOx (nitrogen oxide) contained in the exhaust gas discharged from the internal combustion engine such as the diesel engine or a gasoline engine of a certain type, and various types of combustion units. Among them, there is a NOx occlusion reduction catalyst as a NOx-reducing catalyst for the diesel engine. This NOx occlusion reduction catalyst can be used for effectively purifying the NOx in the exhaust gas.
The NOx occlusion reduction catalyst is formed by a monolithic honeycomb and the like, and is composed by forming plural polygonal cells on a carrier as the structure formed of cordierite, silicon carbide (SiC), or ultra-thin stainless steel of the monolithic honeycomb. A porous catalytic coat layer formed of alumina (Al2O3), zeolite, silica, various types of oxides and the like, which is expected to serve as a catalyst carrier layer, is applied to a wall surface of the cell. A catalyst noble metal (catalytically active metal) which exhibits the oxidizing function and a NOx occlusion agent (NOx occlusion substance: NOx occlusion material: NOx absorbing agent) which exhibits the NOx occlusion function are carried on the surface of the catalytic coat layer. The catalyst noble metal is formed of platinum (Pt) and the like. The NOx occlusion agent is formed of some of the materials selected from the alkali metals such as potassium (K), sodium (Na), lithium (Li), and cesium (Cs), the alkali earth metals such as barium (Ba) and calcium (Ca), the rare earths such as lanthanum (La) and yttrium (Y) and the like. These fulfill three functions of NOx occlusion, NOx release, and NOx purification depending on the oxygen concentration in the exhaust gas.
When the NOx occlusion capacity nearly reaches the saturation by occluding the NOx in the NOx occlusion agent at the time of normal operation, the NOx occlusion reduction catalyst brings the air/fuel ratio of the inflow exhaust gas into a rich state. As a result, the occluded NOx is released, and the released NOx is reduced through the use of a three-way function of the catalyst noble metal.
More specifically, in the case where the air/fuel ratio of the exhaust gas is in the lean state in which oxygen (O2) is contained in the exhaust gas discharged from the ordinary diesel engine and the lean-burn gasoline engine, the oxygen contained in the exhaust gas is used for oxidizing the nitrogen monoxide (NO) discharged from the engine to the nitrogen dioxide (NO2) through the use of the oxidation catalytic function of the catalyst noble metal. The resultant nitrogen dioxide is occluded in the form of the chlorides such as nitrate and the like in the NOx occlusion agent such as barium having NOx occluding function, which results in the purification of the NOx.
But, if the aforementioned state is continued, the NOx occlusion agent having NOx occluding function will be entirely turned into the nitrate, and thus the NOx occluding function is lost. Then excessive rich combustion exhaust gas (rich spike gas) is generated by changing the engine operating condition and injecting the fuel into the exhaust passage so as to be supplied to the catalyst. The excessive rich combustion exhaust gas contains small amounts of coexisting oxygen in the exhaust gas at the high concentration of the reducing agents such as carbon monoxide (CO) and hydrocarbon (HC) and the high exhaust temperature.
In the fuel-rich state where the small amount of oxygen is contained in the exhaust gas at high concentration of the reducing agent and the high exhaust gas temperature, the nitrate having occluded the NOx releases the nitrogen dioxide and returns to the original state, that is, barium or the like. As the oxygen content of the exhaust gas is low, the released nitrogen dioxide is converted into water, carbon dioxide (CO2), and nitrogen (N2) on the catalyst noble metal such as platinum having the oxidizing function by using carbon monoxide, hydrocarbon (HC) and hydrogen (H2) as the reducing agent.
When the NOx occlusion capacity becomes close to saturation, the NOx purification system provided with the NOx occlusion reduction catalyst performs NOx regeneration for supplying reduction composition exhaust gas to the catalyst so as to regenerate the catalyst by releasing the occluded NOx. The reduction composition exhaust gas is generated by increasing the air/fuel ratio of the exhaust gas into the rich state by increasing the fuel amount so as to have the air/fuel ratio higher than the theoretical value to thereby decrease the oxygen concentration of the inflow exhaust gas. The fuel-rich control for recovery of the NOx occlusion capacity is performed to release the absorbed NOx, and the released NOx is reduced by the noble metal catalyst.
In the exhaust gas purification system provided with the NOx occlusion reduction catalyst, the NOx in the exhaust gas is occluded and adsorbed in the NOx occlusion reduction catalyst at the time of the lean combustion in ordinary operation. At the same time, SOx generated by combustion of sulfur contained in the oil or the fuel is also occluded and adsorbed in the NOx occlusion reduction catalyst together with the NOx. The SOx exhibits stronger affinity for the NOx occlusion agent than the NOx to thereby generate the stable compound. Accordingly, the NOx occlusion capacity will be deteriorated as the SOx is occluded, which is the major cause of deterioration in the NOx purification capacity.
Therefore, the engine operating condition is then changed to conduct the sulfur poisoning regeneration control (sulfur purge) for generating the high temperature fuel-rich exhaust gas so as to be supplied to the NOx occlusion reduction catalyst. The SOx is then desorbed and released so as to recover the NOx purification performance of the NOx occlusion reduction catalyst.
Under the sulfur poisoning regeneration control, the high concentration exhaust gas as the uncombusted HC is generated at the temperature around 600°, which may largely deteriorate mileage. Further, the extreme rich condition may deteriorate the state of the exhaust gas, or adversely affect durability of the engine because of to dilution of the fuel oil.
Therefore, the frequency of the sulfur poisoning regeneration control has to be suppressed to a bare minimum. It becomes essential to accurately estimate the deterioration level of the catalyst, that is, the reduction in the amount of the occluded NOx for maintaining the high catalytic performance under the sulfur poisoning regeneration control at the appropriate timing based on the deterioration level of the catalyst derived from accurate decrease in the amount of occluded NOx.
As described in Japanese patent application Kokai publication No. 2002-47919, any of the lambda sensor, the hydrocarbon sensor, the carbon monoxide sensor and the hydrogen sensor is disposed downstream of the nitrogen oxide storage catalyst (corresponding to the NOx occlusion reduction catalyst) to measure the time interval (corresponding to Tx shown in FIG. 2) between switching of the air/fuel mixture (air/fuel ratio) from the lean combustion to the rich combustion (corresponding to T1 shown in FIG. 2), and leakage of the dense exhaust gas passing through the nitrogen oxide storage catalyst (corresponding to T2 shown in FIG. 2). The disclosed method of inspecting the work capacity of the nitrogen oxide storage catalyst uses the resultant time difference Tx for estimating deterioration in the storage capacity of the nitrogen oxide storage material.
In the aforementioned inspection method, the consumption amount of such reducing agents as hydrocarbon, carbon monoxide and hydrogen in the fuel rich exhaust gas composition corresponds to the occluded NOx amount. Thus, the deterioration level of the NOx occlusion reduction catalyst is determined by calculating the NOx occlusion amount based on the time Tx elapsing from the time point T1 at which the air/fuel mixture (air/fuel ratio) is switched from the fuel lean combustion state to the rich combustion state, to the time point T2 at which the aforementioned component leaks to the downstream of the nitrogen oxide storage catalyst.
But, at the initial stage (time Tb shown in FIG. 2) of the rich air/fuel ratio control under the NOx regeneration control in the above-described determination process, the oxygen adsorbed on the surface of the catalyst during the lean operation is released. Thus, the reducing agent such as the hydrocarbon, carbon monoxide and hydrogen detectable by the aforementioned detection sensor is likely to react with the thus released oxygen in preference to the reduction with the NOx. Therefore, the inspection method based on the time Tx which includes the time period Tb (shown in FIG. 2) can fail to provide the accurate value of the NOx occlusion amount.
Patent Document 1: Japanese patent application Kokai publication No. 2002-47919 (paragraph [0036])